On its second to last day as the front-month contract, Brent futures for April delivery were up 16 cents, or 0.3 percent, at $56.15 a barrel by 11:24 a.m. EST (1624 GMT). U.S. West Texas Intermediate crude (WTI), meanwhile, was up 21 cents, or 0.4 percent, to $54.20 per barrel, Reuters said.
Traders said futures pared gains from earlier on Monday after a report from energy data provider Genscape showed a build of more than 800,000 barrels of crude at the Cushing storage hub in Oklahoma, where WTI is priced.
Read alsoReuters: Traders drain pricey U.S. oil storage as OPEC deal bitesInvestors raised their bets on rising Brent crude oil prices to a new high last week, data from the InterContinental Exchange showed on Monday, breaking the 500,000-lot mark for the first time on record.
Money managers also raised their bullish U.S. crude futures and options positions in the week to February 21 to the highest on record, the U.S. Commodity Futures Trading Commission (CFTC) said on Friday.
Investors now hold 951,312 lots' worth of U.S. and Brent crude futures and options, equivalent to nearly 1 billion barrels of oil valued at more than $52 billion, based on current Brent and WTI benchmark prices.
"With speculators increasing their bullish bets on U.S. crude to an all-time high, the risk of disappointment and subsequent downward spiral in prices has never been greater," oil brokerage PVM's Stephen Brennock said.
Among the risks is the level of compliance to the deal between the Organization of the Petroleum Exporting Countries (OPEC) and other producers to bring down oil output by about 1.8 million barrels per day (bpd).
OPEC's record compliance with the deal has surprised the market, and the biggest laggards, the United Arab Emirates and Iraq, have pledged to catch up with their targets.
The International Energy Agency put OPEC's average compliance at a record 90 percent in January. Based on a Reuters average of production surveys, compliance stands at 88 percent.
A Reuters survey of OPEC production later this week will show compliance for February.
"It would appear OPEC has done a commendable job of stabilizing the market. But we are also of the opinion that their intended stability is likely attached to a $60 price handle rather than $50," Jim Ritterbusch, president of Chicago-based energy advisory firm Ritterbusch & Associates, said in a note.
"The longer that values stay below prices needed to drive budget improvement amongst the membership, the greater the likelihood of cheating that could force an eventual unraveling of the OPEC agreement," Ritterbusch said.
Also looming over the success of the OPEC deal is the reaction of U.S. shale producers to rising prices and their ability to increase output.
U.S. drillers added five oil rigs in the week to February 24 to a total of 602, the most since October 2015, energy services firm Baker Hughes Inc said on Friday.